Driver circuit for speeding response of remotely controlled apparatus

ABSTRACT

A fluid motor system controlled by electrical signals originating at a remote location has driver circuits, for energizing motor control valve solenoids in response to signals, with means for assuring fast termination of motor operation upon termination of a control signal. Upon termination of a signal causing motor operation in one direction, the circuit momentarily actuates the motor in the opposite direction. Capacitive means in the driver circuit undergoes a change of charge condition while a control signal is received and thereafter produces a momentary reverse operation signal while reverting to the original charge condition.

United States Patent Coleman et a].

[ Apr. 2, 1974 [54] 3,015,313 1/1962 Faisandier 91/363 A RESPONSE or REMOTELY CONTROLLED w re i elss APPARATUS 3,589,242 6/1971 Peterson et a1 91/413 [75] Inventors: Donald F. Coleman, Dunlap; J e E.

Fuzzen peona both of Primary Examinerlrwin 'C. Cohen [73] Assignee: Caterpillar Tractor Co., Peoria, Ill. Allomey, Agent, y r Tjeflsvoldi Phillips & r Lem io 22 Filed: Aug. 16, 1972 p [21] Appl. No.: 281,214 [57] ABSTRACT Related Apphcauon Data A fluid motor system controlled by electrical signals [62] 13 2 5 62? 1 originating at a remote location has driver circuits, for energizing motor control valve solenoids in responsc' to signals, with means for assuring fast termination of 2% motor operation upon termination of a control signal. i 91/459 361 461 Upon termination ofa signal causing motor operation 1 0 care 1 in one direction, the circuit momentarily actuates the I motor in the opposite direction. Capacitive means in the driver circuit undergoes a change of charge condi- [56] References Cited tion while a control signal is received and thereafter UNITED STATES PATENTS produces a momentary reverse operation signal while 2,649,841 8/1953 Jacques 91 459 x reverting to the original charge condition. 2,701,527 2/1955 Ringoen 318/672 x g Y 2.831%,675 6/l958 Wanlass 307/255 X 1 Claim, 2 Drawing Figures E 52 57 g 45 l 3/ :XTEND l 54 47 FL'U D aim/x1. L SOURCE 37 E 1/ 19 A \14 24 5 E 2/ un- I I5 27 I i g g l -VA I l 7 97 13 u 23 TRANSMITTER RECEIVER 2 l7 29% FLUID I6 1 J 341 34 I 1 CONTRACT SIGNAL L DRIVER CIRCUIT FOR SPEEDING DRIVERCIRCUIT FOR SPEEDING RESPONSE OF REMOTELY CONTROLLED APPARATUS This isa division, of Ser. No. 85,853, filed Nov. 2, 1970, now US. Pat. No. 3,694,671.

BACKGROUND OF THE INVENTION This invention relates to electrical systems for controlling apparatus in response to signals received from a remote location and more particularly to a driver circuit for actuating and deactuating an electrically controlled device in response to such signals.

Certain forms of mechanical apparatus having motors which are controlled directly or indirectly by electrical signals may exhibit a degree of operational inertia in that the motor may not stop immediately upon termination of the control signal which initiated the motor operation. One example of apparatus of this kind is disclosed in copending application Ser. No. 822,368 of Joe E. Fuzzell filed May 7, 1969, now US. Pat. No. 3,606,957, entitled Remote Control System for Load Manipulating Vehicles and assigned to the assignee of the present application. This copending application discloses a loader vehicle, of the form used for manipulating earth or other materials, which may be operated through radio signals originating at a control box some distance away from the vehicle. This is accomplished by providing extensible and contractible fluid cylinders on the vehicle in' the Iinkageswhich connect the operators conventional control levers with the hydraulic valving operated through such levers. By fixing the operators control leversin position and utilizing solenoid operated control valves to extend or contract the fluid cylinders in response to the signals from the remote location, remote control of all the basic vehicle functions is realized.

To deactivate the vehicle mechanism controlled by the cylinders when a remote signal is terminated springs are provided to restore the cylinders to the appropriate degree of extension for stopping the controlled mechanism.

In the absence of corrective measures, a significant period of time may be required for the fluid cylinder to be spring restored to the position at which the associated vehicle mechanism is inactivated. This'produces an undesirable overshoot of the controlled mechanism. Efficient use of the vehicle is made difficult and in some situations a risk of damage or personnel injury may be present.

An experienced observer operating the vehicle from a distance may learn to make proper allowances in the timing of his actuation of the control switches or the like but at best this greatly complicates remote operation of the vehicle and imposes an undesirable burden on the remote operator. Essentially similar problems are encountered in other forms of remotely operated apparatus.

SUMMARY OF THE INVENTION This invention is a circuit for actuating and deactuating mechanism in response to electrical signals wherein the mechanism is stopped extremely rapidly and reliably in response to a terminating signal. A capacitive element in the circuit is arranged to change charge state during receipt of a signal which initiates operation of the controlled apparatus. Upon termination of the actuating signal the capacitive element reverts to the original charge condition thereby momentarily generating a control signal of the form which tends to produce an oppositely directed operation of the controlled apparatus. The reverse control signal has a duration sufficient to rapidly stop the controlled apparatus in its BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a schematic diagram of an electrically controlled fluid motor and an electrical control circuit in accordance with the present invention which responds to radio signals from a remote location; and

FIG. 2 is a schematic diagram of a second embodiment of the invention particularly adapted for controlling a fluid motor with electrical signals transmitted through an umbilical cord containing electrical conductors.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawing and more particularly to FIG. 1 thereof, there is shown a hydraulic motor system 11 typical of mechanisms which it may be desired to control through electrical signals originating at a remote location. The system 11 of this example includes a fluid motor 12 which is a double acting hydraulic cylinder that may be caused to extend or retract or remain at a given position by pivoting an actuator lever 14 of a control valve 13.

Actuator lever 14 has a center position at which both the rod and head ends of motor 12 are blocked and the motor is held in a stationary position. Clockwise movement of lever 14 as viewed in FIG. 1 transmits pressurized fluid from a suitable source 16 to the head end of the motor while venting the rod end to a drain l7. Counterclockwise movement of the actuator lever 14 pressurizes the rod end of the motor 12 while venting the head end to cause contraction.

The system 11 as herein shown is a means for raising and lowering the bucket of a loader vehicle of the form described in the hereinbefore identified copending application Ser. No. 822,368 wherein the valve 13 may be manually controlled by an operator through a control lever 18 connected to the actuator lever 14 through linkage 19. As described to this point, the system 11 is essentially conventional and provides a means whereby the motor 12 is controlled by an operator situated on the vehicle.

As set forth in more detail in the above identified coadjacent stationary structure. Part of the linkage 19 is defined by a fluid cylinder 23 having a head end connected to the control lever and having extensible and contractible rod 24 connected to the actuator lever 14 of valve 13. A pair of compression springs 26 within cylinder 23 act on the piston therein to tend to maintain the cylinder in a state of intermediate extension. Thus, when the operators control lever 18 is immobilized as previously described, springs 26 urge the control valve actuator lever 14 towards the intermediate or Hold position at which motor 12 is stopped.

Owing to the presence of the centering springs 26, cylinder 23 functions effectively as a rigid linkage between control lever 18 and valve actuator lever 14 when the system is operated in the conventional manner by an operator who is present on the vehicle. However, with the operators control lever 18 immobilized, extension of cylinder 23 shifts valve lever 14 to a position causing extension of motor 12 while contraction of cylinder 23 shifts the valve to cause contraction of the motor. Control of cylinder 23 for this purpose in response to signals from the radio transmitter 21 is realized through a pair of solenoid operated valves 27 and 28.

Solenoid valve 27 has an unenergized position at which the head end of cylinder 23 is vented to a drain 29 and an energized position at which a fluid source 31 is coupled to the head end of the cylinder. Similarly solenoid valve 28 has an unenergized position venting the rod end of cylinder 23 to drain 29 and an energized position connecting fluid source 31 with the rod end of the cylinder. Fluid source 31 is shown separate from source 16 in this example of the invention as there are often advantages to using lower pressure fluid for operating cylinder 23 as opposed to motor 12.

Accordingly, with the operators control lever 18 immobilized, motor 12 may be caused to extend by energizing the solenoid 27 of valve 27 and may be caused to contract by energizing the solenoid 28 of valve 28. Upon de-energizing of either solenoid, both ends of cylinder 23 are communicated to drain 29 and springs 26 act to center the actuator lever 14 to stop movement of motor 12. In the absence of corrective measures, this stopping of motor 12 in response to de-energization of a solenoid 27 or 28 may involve an undesirable time delay in that a sizable quantity of fluid must flow from one end or the other of cylinder 23 in response to the pressure of springs 26 before actuator lever 14 is fully returned to the center or Hold position. Thus, the system 11 may not respond instantaneously to the termination of a control signal from transmitter 21 and an undesirable overshoot of the motor 12 may occur. This is avoided in the present invention by a driver circuit 32 which responds to termination of a signal from transmitter 21 not only by de-energizing the particular solenoid 27 or 28' which has been energized but also by momentarily energizing the opposite solenoid to accelerate the centering effect of springs 26 in cylinder 23.

Driver circuit 32 operates from a suitable DC power supply 33 which may be the vehicle battery in this particular example of the invention and has a positive power conductor 34 coupled to a terminal of solenoid 27' through the emitter collector circuit of a power transistor 36 and connected to a terminal of solenoid 28' through the emitter collector circuit of a similar transistor 36'. The other terminal of each solenoid 27 and 28 is grounded and the solenoids 27 and 28 have diodes 37 and 37' connected respectively thereacross through resistors 38 and 38' respectively to protect against inverted transient voltage spikes. Thus either solenoid 27 or 28 may be energized by biasing the associated transistor 36 or 36' to conduction.

Suitable detailed constructions for the transmitter 21 and the receiver 39 are set forth in the hereinbefore identified copending application Ser. No. 822,368. Transmitter 21 is provided with a series of manually operable switches 41 and 41 which may be depressed by a remote operator to initiate extension and contraction respectively of the fluid motor 12. Depression of switch 41 initiates a radio signal which in turn initiates a positive DC electrical signal on an Extend signal conductor 42 of receiver 39 while depression of switch 41 initiates a similar signal on a Contract signal conductor 42' at the receiver. Many additional control signal channels may actually be present at the transmitter 21 and receiver 39 to control a variety of functions, the present description being confined to the two channels for initiating extension or contraction of motor 12 inasmuch as essentially similar circuitry may be utilized to control any number of additional motors 12 or the like.

Considering now the circuitry which renders transistor 36 and 36 conductive in the presence ofa signal on conductors 42 or 42 respectively, the base of each transistor is connected to the positive side of battery 33 through the emitter collector circuit of an associated one of two driver transistors 43 and 43' respectively. The base of each transistor 36 and 36' is also connected to the emitter side thereof through the associated one of the previously described resistors 38 and thus each transistor 36 or 36' is held non-conductive until such time as the associated driver transistor 43 or 43 becomes conductive. Conduction through either of the driver transistors 43 or 43 establishes a voltage drop across the associated resistor 38 biasing the tran- I sistor 36 or 36' into conduction.

Driver transistors 43 and 43 are held non conductive in the absence of signals on conductors 42 and 42 by an associated one of pair of third transistors 44 and 44. The collectors of transistors 44 and 44 are connected to positive power supply conductor 34 through resistors 46 and 46 respectively and the emitters of the two third transistors are grounded through resistors 47 and 47 respectively. The collectors of transistors 44 and 44' also connect to the bases of driver transistors 43 and 43 respectively through additional resistors 48 and 48. Resistors 47 and 47 have relatively small values in comparison with resistors 46 and 46' and therefore a potential close to ground is provided to the base of transistors 43 and 43', holding the transistors non-conductive, as long as the third transistors 44 and 44' are conductive. The third transistors 44 and 44' are held conductive, in the absence of input signals on conductors 42 and 42, through a base connection to positive battery through resistors 49 and 49 respectively and a base connection to ground through resistors 51 and 51' respectively. Resistors 49 and 51 and resistors 49' and 51 thus constitute voltage dividers normally biasing the bases of third transistors 44 and 44 respectively to establish conduction therethrough.

Signal conductors 42 and 42' connect to ground through resistors 52 and 52' respectively and also through a parallel circuit defined by resistors 53 and 54 in the case of output conductor 42 and resistors 53 and 54' in the case of output conductor 42. A capacitor 56 is connected between the base of transistor 44 and the junction between resistors 53 and 54 and a similar capacitor 56 is connected between the base of transistor 44' and the junction between resistors 53' and 54. The Extend signal conductor 42 also connects, through a diode 57 and resistor 58 with the base of transistor 43 while contract signal conductor 42 similarly connects with the base of transistor 43 through a diode S7" and resistor 58'.

In operation, with no signals on conductors 42 and 42', power transistors 36 and 36 are non-conductive for the reasons hereinbefore described and thus solenoids 27' and 28 are unenergized and the motor control valve lever 14 is therefore in the centered position immobilizing the motor 12. Under this condition capacitors 56 and 56 have a small positive charge due to the connection of the capacitors to the voltage dividers defined by resistors 49 and 51 and 49 and 51'. When, for example, an Extend signal is received on signal conductor 42 the signal is transmitted through diode 57 to the base of, transistor 43' causing transistor 43 to conduct and thereby provide base drive to power transistor '36. Power transistor 36' saturates and closes the circuit from the positive side of battery. 33 to ground through solenoid 28' thereby energizing the solenoid and, operating valve 28. Valve 28 then applies fluid pressure to the rod end of cylinder 23 which retracts to pull control valve 13 actuator lever 14 clockwise as viewed in FIG. 1. Fluid from source 16 is thereby supplied to the head end of motor 12 causing the motor to extend. I

During the period that the Extend signal is present on signal conductor 42, the charge on capacitor 56 reverses polarity and rises to a higher level than was formerly the case.

When the Extend signal on conductor 42 is terminated, base bias through diode 57 and resistor 58 is removed from transistor 43'fTransistor 43 turns off thereby removing base bias from power transistor 36' and opening the circuit between battery 33 and solenoid 28. Valve 28 then reverts to the de-energized position at which the rod end of cylinder 23 is opened to drain- 2 9. Sprin'gs26 are thus enabled to begin centering the piston of cylinder 23 to restore control valve 13 actuator lever 14 to the intermediate position to immobilize motor 12.

As immobilization of motor 12 in this manner re-' quires that a finite volume of fluid be expelled from cylinder 23 through the actions of springs 26, some sizable delay would occur, in the absence of corrective measures, before control valve 13 is fully restored to the Hold position. Considering now the means by which the present invention reduces this delay to an insignificant time period, the termination of the extend signal onconductor 42 acts, in addition to the effects described above, to enable capacitor 56 to begin to discharge at a rate determined by the values of the resistors 52, 53, 54 and 49 as well as the value of the capacitor itself. As the capacitor 56 is discharging, the potential at the base of transistor 44 decreases shutting off transistor 44. Loss of conduction through transistor 44 causes a positive voltage rise at the base of transistor 43 which becomes conductive and provides base drive to power transistor 36 which becomes conductive in turn energizing the other solenoid 27. This energizing of solenoid 27 is momentary in that the circuit reverts to the original condition when capacitor 56 is restored to the original condition.

The brief energizing of solenoid 27 following termination of the Extend signal operates valve 27 to momentarily direct high pressure fluid to the rod end of cylinder 23 thereby accelerating the action of springs 26 in restoring the cylinder to the centered position at which control valve 13 is in the Hold position and motor 12 is immobilized.

The driver circuit 32 operates in a similar manner in from a remote location by the modified driver circuit 32 of FIG. 2 may be similar to that described with ref erence to previous embodiments and thus includes a hydraulic motor 12 operated by pressurized fluid from a suitable source 16 by manipulation of an actuator arm 14 ofa control valve 13'. Under non-remote control conditions, actuator lever 14 is operated by pivoting a control lever 18' through linkage 19' connecting the two levers. To provide for optional remote control, a small fluid cylinder 23 defines a portion of the linkage 19' between levers 18' and 14' and contains centering springs 26 which cause the cylinder to function effectively as a rigid link when control lever 18 is pivoted. Cylinder 23 may be extended for remote control purposes by energizing a solenoid valve 28 controlled by a solenoid 28" and the cylinder may be caused to contract by energizing an additional solenoid valve 27 having a control solenoid 27". Energization of solenoid 28" causes pressurized fluid from a source 31' to be transmitted to the rod end of cylinder 23 while energization of solenoid 27" causes fluid from source 31 to be directed to the headend of cylinder 23. The detailed construction and operation of the above described fluid system components of the apparatus of FIG. 2 may be identical to that of the system 11 previously described with reference to the first embodiment of the apparatus.

A suitable DC power supply such as a battery 33' has the negative side grounded and has a positive side connected by a powerconductor 34 with one terminal of each solenoid 27. and 28" through the emitter collector circuits of power transistors 61 and 61' respectively. Thus solenoid 27" connects with battery 33' through the emitter collector circuit of transistor 61 and solenoid 28" connects with battery 33' through the emitter collector circuit of transistor 61. To guard against inverted voltages, diodes 62 and 62' are disposed between the battery 33' and transistors 61 and 61' respectively. The other terminals of both solenoids 27" and 28" are grounded and each has a protective diode 63 connected thereacross in series with resistors 64 and 64' respectively. Accordingly solenoid valve 27" or valve 28 may be selectively operated by biasing transistor 61 or 61 respectively into conduction.

To control the power transistors 61 and 61 in response to signals from a remote location, driver circuit 32' has an Extend signal conductor 66 and a Contract signal conductor 66' which, together with a ground potential conductor 67, extend through an umbilical cord 68 to a remote control unit 69 which may be situated at any desired distance from the remainder of the apparatus.

The base of power transistor 61 is connected to Extend signal conductor 66 through a resistor 71 and protective diode 72 while the base of transistor 61 is similarly connected to Contract signal conductor 66 through a resistor 71' and diode 72'. Remote control unit 69 has a first normally open, manually operable switch 74 for selectively connecting Extend signal conductor 66 to ground conductor 67. Grounding of signal conductor 66 establishes a voltage drop across resistor 71 which biases power transistor 61 into conduction thereby energizing valve solenoid 27". Similarly, the remote control unit 69 has a second normally open manually operable switch 74 for selectively grounding signal conductor 66' to bias power transistor 61- into conduction and thereby energize valve solenoid 28".

The driver circuit 32 of FIG. 2 as described to this point provides a means by which hydraulic motor 12' may be selectively caused to extend to contract by operation of switches 74 and 74' respectively. As hereinbefore discussed an undesirable delay in response of motor 12 to termination of a control signal may occur in the absence of corrective measures. As in the previous instance, the modified driver circuit 32' provides for a brief energization of a solenoid 27" or 28 following termination of energization of the other solenoid so that restoration of cylinder 23 to the intermediate position is assisted by fluid pressure from source 31.

To perform this function, the signal conductors 66 and 66' are'connected to ground through the emitter collector circuits of a pair of signal end detector transistors 76 and 76 respectively and through a single diode 77. Thus either signal conductor 66 or 66' may be grounded to turn on the associated power transistor 61 or 61' by biasing the associated transistor 76 or 76 into conduction, the effect being similar to closing of one of the manually operated switches 74 and 74 at the remote control unit 69.

The base of transistor 76 connects to the positive side of battery 33 through a capacitor 78 and resistor 79 while the base of transistor 76' is similarly connected to the positive side of the battery through capacitor 78 and resistor 79. Extend signal conductor 66 is connected to the junction 81 between capacitor 78' and resistor 79 while contract signal conductor 66' is simi' larly connected to the junction 81 between capacitor 78 and resistor 79. Diodes 82 and 82' are connected between ground and the bases of transistors 76 and 76' respectively.

During the period when both maually operated switches 74 and 74 at the remote control unit are open, capacitors 78 and 78 are charged owing to the connection to battery 33' through resistors 79 and 79 respectively. With capacitors 78 and 78 fully charged and switches 74 and 74' opened, no base bias is present at transistor 76 and 76 and both such transistors are turned off. Upon closing of switch 74, for example, extend signal conductor 66 is grounded as previously described causing energization of solenoid 27" by the resultant conduction through power transistor 61. This closing of switch 74 also provides a discharge path for capacitor 78 owing to the connection between extend signal conductor 66 and junction point 81. When switch 74 is subsequently opened to de-energize solenoid 27", the ground connection to terminal point 81' is removed and capacitor 78 begins to recharge. During the period that capacitor 81 is in the process of recharging, transistor 76 becomes conductive. Conduction through transistor 76 is brief as the base bias thcreat exists only while capacitor 78' is charging and ends when a steady charge condition is reestablished in the capacitor. During this brief period at which transistor 76 conducts, power transistor 61 is also biased into conduction to briefly energize solenoid 28". This momentarily operates valve 28 to cause pressurized fluid from source 31 to accelerate the action of springs 26 in cylinder 23' in restoring control valve 13' to the Hold position at which motor 12' is immobilized.

Closing of switch 74 at remote control unit 69 and the eventual reopening of switch 74' produces an essentially similar effect. When switch 74 is reopened, solenoid 28" is de-energized as previously described. The reopening of switch 74' also causes capacitor 78 to recharge and to turn on transistor 76 during the brief recharging period. During the period that transistor 76 is conductive, solenoid 27" is energized enabling the force of fluid from source 31 to accelerate restoration of control valve 13 to the Hold position.

Thus the circuit depicted in FIG. 2 differs from that of FIG. 1 in that no transistors are conductive in the absence of a control signal. Control signals are generated by the grounding of the signal conductors 66 and 66 rather than the application of positive voltage thereto. Capacitors 78 and 78 generate a brief inverse mode of valve'operation, following termination of a control signal, while charging rather than while discharging. Thus many variations are possible witin the scope of the invention and it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In combination, a circuit for controlling electrically actuated mechanism in response to signals of se lectively variable duration from a remote location, said mechanism having a first terminal at which electrical voltage is changed to provide a forward mode of operation for a selected period and having a second terminal at which electrical voltage is changed to provide a reversed mode of operaton for a selected period, wherein said electrically actuated mechanism comprises an extensible and contractible fluid motor, a control valve for said motor having actuator means movable between three positions including a position at which said motor is caused to extend in said forward mode of operation and a position at which said motor is caused to contract in said reversed mode of operation and a position at which said motor is immobilized, a fluid cylinder connected to said control valve actuating means and being extensible and contractible to shift said actuating means between said three positions thereof and having resilient means acting thereon tending to urge said cylinder to the position at which said actuator means immobilizes said fluid motor, an electrically operated valve for directing fluid to said cylinder to cause said cylinder to shift said control valve actuating means to the motor extending position thereof controlled by said first terminal, a second electrically operated valve for directing fiuid to said cylinder to cause said cylinder to shift said control valve actuating means to the motor contracting position thereof controlled by said second terminal, said circuit comprising:

a first signal conductor for receiving signals from said remote location for initiating operation of said mechanism in said forward mode thereof,

means for changing voltage at said first terminal in response to a signal on said first signal conductor and for maintaining said voltage change for the duration of said signal,

a second signal conductor for receiving signals from said remote location for initiating operation of said ate said mechanism in said forward mode. 

1. In combination, a circuit for controlling electrically actuated mechanism in response to signals of selectively variable duration from a remote location, said mechanism having a first terminal at which electrical voltage is changed to provide a forward mode of operation for a selected period and having a second terminal at which electrical voltage is changed to provide a reversed mode of operaton for a selected period, wherein said electrically actuated mechanism comprises an extensible and contractible fluid motor, a control valve for said motor having actuator means movable between three positions including a position at which said motor is caused to extend in said forward mode of operation and a position at which said motor is caused to contract in said reversed mode of operation and a position at which said motor is immobilized, a fluid cylinder connected to said control valve actuating means and being extensible and contractible to shift said actuating means between said three positions thereof and having resilient means acting thereon tending to urge said cylinder to the position at which said actuator means immobilizes said fluid motor, an electrically operated valve for directing fluid to said cylinder to cause said cylinder to shift said control valve actuating means to the motor extending position thereof controlled by said first terminal, a second electrically operated valve for directing fluid to said cylinder to cause said cylinder to shift said control valve actuating means to the motor contracting position thereof controlled by said second terminal, said circuit comprising: a first signal conductor for receiving signals from said remote location for initiating operation of said mechanism in said forward mode thereof, means for changing voltage at said first terminal in response to a signal on said first signal conductor and for maintaining said voltage change for the duration of said signal, a second signal conductor for receiving signals from said remote location for initiating operation of said mechanism in said reversed mode thereof, means for changing voltage at said second terminal in response to a signal on said second signal conductor, and for maintaining said voltage change for the duration of said signal, and means for responding to termination of each signal on said first signal conductor by briefly changing the voltage at said second terminal to momentarily operate said mechanism in said reversed mode and responding to termination of each signal on said second signal conductor by briefly changing the voltage of said first terminal to momentarily operate said mechanism in said forward mode. 